Various methods for extracorporeal blood treatment are known. In hemodialysis (HD), the patient's blood is cleaned in an extracorporeal blood circuit which comprises a dialyzer. The dialyzer comprises a blood chamber and a dialyzing fluid chamber, which are separated by a semipermeable membrane.
Whereas dialyzing fluid flows through the dialyzing fluid chamber in the case of hemodialysis (HD), wherein specific substances are transported through the membrane due to diffusion between the dialyzing fluid and the blood, dialyzing fluid does not flow through the dialyzing fluid chamber of the dialyzer in the case of hemofiltration (HF). In the case of hemofiltration (HF), specific substances are effectively removed through the membrane of the filter due to convection. Hemodiafiltration (HDF) is a combination of the two methods.
It is generally known to replace a part of the fluid withdrawn from the patient via the membrane of the dialyzer or filter by a sterile substitution fluid (substituate), which is fed to the extracorporeal blood circuit upstream or downstream of the dialyzer. Apparatuses for extracorporeal blood treatment are known in which the dialyzing fluid is prepared online from fresh water and concentrate and the substituate is prepared online from the dialyzing fluid. The substituate is fed to the extracorporeal blood circuit from a dialyzing fluid system of the blood treatment apparatus via a substituate line upstream or downstream of the blood chamber of the dialyzer.
In order to avoid possible contamination of the dialyzing fluid with poisonous substances (pyrogens), the dialyzing fluid is filtered before it is fed to the blood circuit. For this purpose, the known blood treatment apparatuses comprise special pyrogen filters, which effectively retain decomposition products of bacteria and endotoxins in order to maintain a pyrogen-free dialysate (permeate).
The known blood treatment apparatuses generally comprise one or more pyrogen filters connected in series, which are disposed in the fluid system. The pyrogen filters have a similar structure to the dialyzer. Pyrogen filters are known which comprise a bundle of hollow fibers. Such pyrogen filters are referred to as capillary filters. The hollow-fiber bundle is disposed in a cylindrical housing in such a way that the openings of the individual fibers lie free at the end faces of the bundle. The capillaries in the hollow fibers form the primary side and the intermediate space between the hollow fibers forms the secondary side of the filter. The filtering takes place diffusively due to the pressure difference transversely to the fiber direction.
It is possible to subject pyrogen filters to an initial pressure test with compressed air. Here, the filter is closed downstream by a valve and an air overpressure is applied upstream and the pressure is monitored. If the pressure remains unchanged over a certain period of time, it can be assumed that there is an intact filter. If the pressure drops, it can thus be concluded that there are leaks in the fibers through which the compressed air can escape.
The known pyrogen filters have to be replaced after a certain time in service. The drawback is that, in practice, the replacement intervals for pyrogen filters have to be rigidly preselected irrespective of the actual state of the filter. In practice, therefore, the filters may in principle be changed too soon or too late.
US 2008/0203023 A1 describes a blood treatment apparatus, which comprises a pyrogen filter which comprises an integrated conductivity sensor, in order to monitor the quality of the dialyzing fluid flowing through the filter. This alone does not however allow information to be provided as to the state of the filter.
WO 2008/089913 A2 describes a blood treatment apparatus, which comprises a device for establishing the calcification of a pyrogen filter. This device comprises sensors which are disposed upstream and downstream of the filter. The sensors measure the conductivity of the dialyzing fluid flowing continuously through the semipermeable membrane of the filter.
DE 198 32 451 C1 describes a device for checking the correctly implemented replacement of a used filter in an apparatus for extracorporeal blood treatment. The checking of the correctly implemented filter replacement takes place by means of a pressure-holding test, wherein it is concluded, in the case of permeability of the filter membrane for a gas, that the used filter has been replaced by a new filter.